Process for preparing a silica suspension in a curable silicone in order to form elastomers

ABSTRACT

The invention concerns the preparation of a silica suspension in a silicon fluid, said suspension being used to produce silicon vulcanisable by polyaddition (RTV elastomers). The problem which the invention aims to solve is that of finding a technical compromise between the rheological and mechanical properties of the final RTV. The invention solves the problem by providing a method for preparing a silica suspension treated with hexamethyldisilazane (HMDZ) in a silicon fluid with siloxyl Si-Vinyl function, characterised in that it consists essentially in introducing HMDZ in the preparation medium, before and/or substantially while bringing at least part of the silicon fluid prepared with at least part of the particulate filler used, said introduction being carried out once or several times for a HMDZ fraction corresponding to a proportion not more than  8 % by dry weight with respect to the total silica charge and after bringing together the POS and the filler.

TECHNICAL FIELD:

[0001] The field of the invention is that of silicone elastomers whichcan be obtained by polyaddition and the main components of which aresilicone polymers and fillers.

[0002] More specifically, the present invention relates to thepreparation of an intermediate product useful for obtaining thesesilicone elastomers and consisting of a suspension of a reinforcingfiller in a polyorganosiloxane carrying Si-alkenyl—preferablySi—Vi—functional groups capable of reacting by polyaddition with the SiHcrosslinking functional groups of another POS.

PRIOR ART

[0003] A distinction may be made between reinforcing and non-reinforcingfillers in silicone rubbers.

[0004] The most widely used reinforcing fillers are preferably pyrogenicsilicas having a BET surface area>50 m²/g. They owe their reinforcingeffect firstly to their morphology and secondly to the hydrogen bondswhich form between the silanol groups on the surface of the silicas(3-4.5 SiOH groups/mm²) and the polyorganosiloxane (POS) chains. Theseinteractions between the filler and the polymer increase the viscosityand modify the behaviour of the polymer near the solid surface of thefillers. Moreover, the bonds between polymers and fillers improve themechanical properties but may also cause prejudicial premature curing(“structuring”) of the precursor compositions of the elastomers.

[0005] Non-reinforcing fillers interact extremely weakly with thesilicone polymer. These are, for example, chalk, quartz powder,diatomaceous earth, mica, kaolin, aluminas or iron oxides. Their effectis often to increase the viscosity of the uncured precursors of theelastomers, as well as the Shore hardness and the modulus of elasticityof these precursors.

[0006] Silicone elastomers may also contain, inter alia, catalysts,inhibitors, crosslinking agents, pigments, antiblocking agents,plasticizers and adhesion promoters.

[0007] These elastomers, curable by polyaddition and also called RTVelastomers, are formed, before curing, by casting, extrusion,calendering, or compression, injection or transfer moulding.

[0008] Silicone compositions made of elastomers, which can be cured bypolyaddition at room temperature or at higher temperatures (generally<200° C.), are conventionally packaged in the form of two-componentsystems, that is to say comprising two parts which are packagedseparately and have to be mixed at the time of use.

[0009] In two-component systems, one of the components comprises thecatalyst for the polyaddition reaction. This catalyst is preferably ofthe platinum kind. It may, for example, be a platinum complex like theone prepared from chloroplatinic acid and1,3-divinyl-1,1,3,3-tetramethyldisiloxane, according to U.S. Pat. No.3,814,730 (Karstedt catalyst). Other platinum complexes are described inU.S. Pat. Nos. 3,159,601, 3,159,662 and 3,220,972.

[0010] This component including the catalyst may also comprise only oneof the POS fluids of type I having Si-alkenyl, preferably Si-vinyl,crosslinking functional groups or only one of the POS fluids of type IIhaving an SiH crosslinking functional group.

[0011] Generally, the POS fluids of type I and the POS fluids of type IIcomprise at least two Si—Vi and SiH groups per molecule, respectively,preferably in the α and ω positions on the chain: at least one of thetwo having to comprise at least three crosslinking functional groups permolecule.

[0012] These compositions comprise, in a known manner, POS fluids oftype I and II, a platinum catalyst for crosslinking by polyaddition anda platinum inhibitor allowing the compositions to cure only once theyhave been removed from the package and mixed together, optionally afterthey have been heated slightly.

[0013] As examples of inhibitors, mention may be made of:

[0014] polyorganosiloxanes, advantageously cyclic polyorganosiloxanes,substituted with at least one alkenyl, tetramethylvinyltetrasiloxanebeing particularly preferred,

[0015] pyridine,

[0016] organic phosphines and phosphites,

[0017] unsaturated amides,

[0018] alkylated maleates

[0019] and acetylenic alcohols (cf. FR-B-1,528,464 and FR-A-2,372,874).

[0020] Such compositions may also be in the form of one-componentsystems which cure only after having been heated.

[0021] The preparation of concentrated suspensions (pastes) ofreinforcing silicas in vinyl silicone oils, these suspensions beingintended to produce elastomers that can be cured by the reaction of apolyhydrogenated crosslinking molecule such as a POS with the vinylsilicone oil (SiH/SiVi addition), is widespread in the field ofelastomers.

[0022] The commonest reinforcing particulate fillers are based onsilica, but substances such as TiO₂, Al₂ O₃ and kaolin, for example, mayalso be used in certain cases.

[0023] These reinforcing fillers have a BET specific surface area of atleast 50 m²/g, and generally up to 400 m²/g. These are ultrafine powderswhich may be dispersed in silicone, preferably SiVi, oils. Thisdispersion causes problems when mixing some of the pulverulent fillerwith the oil and particular care must be taken in order to obtain auniform distribution of the fillers in the suspension.

[0024] Another difficulty to be overcome is associated with the rheologyof the suspensions prepared. This is because it is clear thatintroducing a pulverulent particulate filler of very small particle sizeinto the silicone oil necessarily causes an appreciable increase in theviscosity. However, this characteristic, although it accompanies theachievement of good mechanical properties for the silicon elastomerscomprising the suspension as raw material, is prejudicial to thehandling and forming of the suspension and of the silicone compositionscontaining the suspension. It is in fact more convenient, for moulding,extrusion or forming, to handle fluid compositions which readily lendthemselves, inter alia, to pumping, flowing or mixing with functionaladditives.

[0025] The problematic considered here may therefore be summarized ashow to find a technical compromise between a priori antinomicspecifications for the suspensions of fine particulate fillers insilicone oils, namely: fine distribution of particles in the siliconematrix—uniformity of the dispersion—suitability of the rheology of thesuspension to the handling constraints (processibility)—mechanicalproperties of the RTV silicone elastomers.

[0026] French Patent Application No. 2,320,324 falls within thisproblematic and describes a process for a homogeneous distribution inpolyorganosiloxanes of a highly disperse active filler of BET specificsurface area of at least 50 m²/g, this process being characterized inthat the filler is treated during incorporation, in the presence ofwater, by a modifier or compatibilizer of the silazane type,hexamethyldisilazane being particularly preferred. The othercompatibilizers mentioned are trimethylsilane, trimethylchlorosilane,trimethylethoxysilane, triorganosilyl mercaptans, triorganosilylacylates or triorganosilyl amines. According to this process, describedin this prior application, an α, Ω-trimethylsiloxy polydimethylsiloxanewith hexamethyldisilazane (HMDZ) and with water. Once this mixture hasbeen homogenized, some particulate silica is incorporated into it andmixing is continued until a homogeneous mixture is obtained. Next, themixture is heated to 130° C. in order to remove the excess HMDZ andwater by devolatilization. It is left to cool and, after measuring theviscosity of the suspension obtained, it is found that the latter isrelatively high, which, of course, gives the elastomers capable of beingprepared from this suspension good mechanical properties, but whichproves to be unacceptable from a handling standpoint in an industrialcontext. This compatibilization treatment of the silica with thesilicone oil may be termed “early” since the HMDZ is present as soon asthe reinforcing silica is brought into contact with this silicone oil.

[0027] The compatibilization treatment is a means of retarding orpreventing reaction between the surface of a reinforcing filler and asiloxane polymer. This interaction causes what is called structuring andas a result the conversion of these mixtures is more difficult.

[0028] Processes for preparing a suspension of reinforcing silica insilicone oils are also known, in which the compatibilization treatmentwith the aid of hexamethyldisilazane is carried out after the silica hasbeen incorporated into the silicone oil. This method of treatment istermed here “late”. It provides relatively fluid suspensions which mayhave a tendency to structure over time. In certain cases, thesuspensions formed may have a certain tendency to be thixotropic. Thisis not without having undesirable consequences when converting andhandling these suspensions, especially when degassing them.Nevertheless, it may be stated that, whatever the rheological propertiesof the suspensions obtained by late HMDZ treatment, the final mechanicalproperties of the elastomers prepared from the said suspensions areperfectible (hardness, tensile strength, elongation at break, tearstrength). In particular, they do not reach the level of those found inthe case of early treatment.

[0029] By way of illustration of this type of compatibilizationtreatment, mention may be made of European Patent Application No.0,462,032 which describes a process for preparing a paste which can beused especially in compositions that are curable by a polyadditionreaction and which thus allows silicone elastomers to be obtained.According to this process, the following are injected, continuously andsimultaneously, into a twin-screw extruder, at at least four differentpoints:

[0030] at least one SiVi POS oil,

[0031] water,

[0032] silica,

[0033] a liquid polysilazane under normal temperature and pressureconditions (HMDZ).

[0034] The silica is injected downstream of the water and of the oil andupstream of the HMDZ without, however, there being any mixing, betweenthe HMDZ and water on the one hand and between the HMDZ and the silicaon the other, before introduction of HMDZ.

[0035] U.S. Pat. No. 4,785,047 discloses a hybrid compatibilizationtreatment, at the boundary between the early and late treatmentsmentioned above. This patent relates more specifically to a process forpreparing transparent silicone elastomers. This document describespumpable liquid compositions formed from suspensions of HMDZ-treatedsiliceous filler in silicone oils which may or may not carrypolyaddition-crosslinking functional groups (SiH/SiVi). The problematicpresented in this patent is different from that of the prior artpresented above. In this case it is in fact more one of obtainingtransparent elastomers and, in order to do so, of trying to counteractthe deleterious effect of the siliceous filler on the transparency by avery substantial HMDZ treatment. According to the process forming thesubject of that patent, part of the silicone oil is firstly mixed withall of the water and all of the silica, but only with a fraction of theHMDZ representing systematically more than 15% by dry weight withrespect to the silica, namely 34% and 26% in the examples.

[0036] After this first mixture has been homogenized, the remaining HMDZis incorporated and mixed into the latter.

[0037] Next, the devolatilization treatment is carried out for 1 hour at150° C. and under reduced pressure.

[0038] Finally, the rest of the PMDS silicone oil and the α,ω-diVi PDMSsilicone oil are mixed for 1 hour at room temperature.

[0039] The transparent curable silicone suspension obtained has aviscosity lying between 200 and 10,000 Pa.s at 25° C.

[0040] This technical proposal may possibly provide a solution to thetransparency problem, but it proves to be unsatisfactory with regard tothe viscosity of the suspension and to its handling.

BRIEF SUMMARY OF THE INVENTION

[0041] In such a technical context, one of the essential objectives ofthe present invention is to provide a process for preparing a suspensionof a particulate filler, treated with the aid of a compatibilizer, in asilicone oil, this suspension being able to be used as a raw materialfor the production of RTV elastomer compositions that can be cured bypoplyaddition.

[0042] This process has to meet the following specification:

[0043] the distribution of the filler in the silicone oil must beuniform and homogeneous,

[0044] the dispersion must be optimal,

[0045] the suspension must flow well (no flow threshold) and theviscosity must be suitable for handling and converting the suspension,

[0046] the mechanical properties of the elastomers must be of anacceptable level.

[0047] Another essential objective of the invention is to provide aprocess for preparing a reinforcing filler/silicone oil suspension forRTV elastomers which is simple to employ, inexpensive and able to beapplied on an industrial scale.

[0048] Another essential objective of the invention is to provide aprocess for obtaining a silicone composition, curable by polyaddition inorder to form an RTV elastomer and comprising, as a constituent element,the suspension as obtained by the intended process above.

[0049] These objectives, among others, are achieved by the presentinvention which relates to a process for preparing a suspension of aparticulate, preferably siliceous, filler in a material formed by asilicone oil comprising:

[0050] polyorganosiloxanes (POS fluids) of type (I) which carrySi-alkenyl—preferably Si-vinyl—functional groups capable of reactingwith the Si—H crosslinking functional groups of a POS fluid of type II,

[0051] optionally, POS fluids of type (II) which carry Si—H crosslinkingfunctional groups capable of reacting with the Si-alkenyl functionalgroups of the POS fluids (I),

[0052] and/or, optionally, POS fluids of type (III) which differ fromthe POS fluids (I) and (II),

[0053] the said suspension being able to be used, in particular, forproducing silicone compositions that can be cured by polyaddition,

[0054] this process being of the kind of those in which the particulatefiller is treated with the aid of a compatibilizing agent orcompatibilizer (CA),

[0055] characterized in that it essentially consists in introducing somecompatibilizer (CA) into the preparation mixture:

[0056] on the one hand, before and/or substantially simultaneously withthe contacting of at least part of the silicone oil employed with atleast part of the inarticulate filler used, this CA introduction takingplace in one or more steps for a CA fraction corresponding to aproportion of at most 8%, preferably at most 5% and even more preferablyat most 3% by dry weight with respect to the total particulate filler;

[0057] and, on the other hand, after this POS/filler contacting.

[0058] It is to the credit of the inventors that they have demonstrated,after extensive research and many experiments, that it is surprisinglyand unexpectedly appropriate to incorporate the compatibilizer (forexample HMDZ) before and after the reinforcing, preferably siliceousfiller has been mixed with the silicone oil, (preferably of SiVi type(I)), as long as the fraction of compatibilizer CA introduced beforePOS/filler mixing corresponds to less than 5% by weight of the totalreinforcing filler.

[0059] These novel and advantageous provisions make it possible toobtain suspensions having suitable rheological properties and suitableviscoelastic behaviour. This is because these suspensions do not have aflow threshold, or have a very low threshold which is not prejudicial tothe applications. This considerably improves their processing.

[0060] In particular, they have a fluidity which is stable over time andsuitable for the handling and conversion operations, such as pumping,transferring, mixing, forming, moulding, extrusion, etc.

[0061] One of the major advantages of the invention is that thisattainment from the rheology standpoint is not to the detriment of thefinal mechanical properties of the crosslinked elastomer. The technicalcompromise is achieved.

[0062] Moreover, the methodology adopted makes it possible to obtaingood homogeneous dispersions of the particulate filler in the oil. Inaddition, this methodology does not significantly complicate theprocess, which remains simple and inexpensive to implement.

DETAILED DESCRIPTION OF THE INVENTION

[0063] In accordance with one of these preferred methods ofimplementation, the process according to the invention essentiallyconsists:

[0064] in mixing:

[0065] 100 parts by weight of silicone oil

[0066] 0 to 5 parts by weight of water

[0067] 20 to 80 parts by weight of particulate filler consisting ofsilica

[0068] 1 to 20 parts by weight of compatibilizer (CA) selected fromsilazanes taken by themselves alone or as a mixture thereof, preferablyfrom disilazanes, hexamethyldisilazane which may or may not be combinedwith divinyltetra-methyldisilazane being particularly preferred;

[0069] in leaving the above to react, preferably with stirring,

[0070] in heating the mixture obtained, choosing a pressure/temperaturepair so that at least some of the water and of the volatile elementsundergoes devolatilization;

[0071] if necessary, in cooling the mixture.

[0072] In other words, the process according to the invention makes itpossible to control the viscosity of the suspension while at the sametime maintaining the mechanical properties of the final elastomerobtained from the suspension at an acceptable level, or even improvingthis level.

[0073] The mixing is carried out with the aid of known and suitabledevices. These may be, for example:

[0074] arm mixers

[0075] internal mixers

[0076] planetary mixers

[0077] ploughshare mixers

[0078] corotating or counterrotating twin-shaft mixers

[0079] continuous extruder-mixers

[0080] or other continuous or batch devices.

[0081] The mixing operation is carried out at normal temperature andpressure and preferably in an inert atmosphere (N₂). Moreover, underthese conditions the silicone oil, the water but also the compatibilizerare in liquid form in order to make the mixing easy.

[0082] The reinforcing, preferably siliceous, filler represents from 10to 50% by weight of the suspension. In practice, this filler is of theorder of 30±10%.

[0083] Advantageously, the proportion of compatibilizer introduced in afirst step is at most equal to 8% of the reinforcing filler (and, forexample, between 1 and 3% of the reinforcing filler, preferably between1 and 2%). Moreover, it may be pointed out that the total amount ofcompatibilizer CA is preferably between 5 and 30% of the siliceousfiller, preferably between 10 and 20%.

[0084] The proportions of compatibilizer AC introduced before and afterfiller/oil mixing are (5-25), preferably (10-20%), respectively.

[0085] In order to define more precisely the preferred method ofimplementing the process according to the invention, without howeverthis being limiting, it may be pointed out that the process comprisesthe following steps:

[0086] all or some of the silicone oil, the water and all or some of theparticulate siliceous filler are mixed with a first CA fraction ofbetween 1 and 3% by dry weight with respect to the silica,

[0087] a second CA fraction, representing between 10 and 15% by dryweight of silica, is incorporated into the mixture,

[0088] optionally, the rest of the silicone oil and the rest of thesilica are added,

[0089] the mixture is allowed to react, preferably by continuing themixing,

[0090] the mixture is devolatilized, preferably in an inert-gasatmosphere (eg. N₂),

[0091] optionally, the devolatilized mixture is allowed to cool

[0092] and, optionally, the suspension is completed with the rest of thesilicone oil.

[0093] According to a first particular practical implementation of theprocess of the invention, it comprises the following steps:

[0094] 1. a mixture comprising the silicone oil, the water and the firstCA—preferably HMDZ—fraction is homogenized,

[0095] 2. the particulate filler, preferably silica, is gradually addedto the mixture obtained at 1,

[0096] 3. the mixing is continued,

[0097] 4. the second CA—preferably HMDZ—fraction is graduallyincorporated into the mixture obtained at 3,

[0098] 5. the mixing is continued,

[0099] 6. the mixture is devolatilized, preferably by heating to atemperature≧100° C.

[0100] In step 1 of this first practical implementation, a choice ismade between, inter alia, the following three alternatives:

[0101] a) either all the oil and all the silica, as well as the initialCA fraction, are used,

[0102] b) or all the oil, part of the silica and the initial CA fractionare used,

[0103] c) or all the silica, part of the oil and the initial CA fractionare used.

[0104] According to a second particular practical implementation of theinvention, it comprises the following steps:

[0105] 1′. the silicone oil and the water are homogenized,

[0106] 2′. the particulate filler—preferably silica—and, at the sametime, the first CA—preferably HMDZ—fraction are gradually incorporatedinto the mixture obtained at 1,

[0107] 3. the mixing is continued,

[0108] 4. the second CA—preferably HMDZ—fraction is graduallyincorporated into the mixture obtained at 3,

[0109] 5. the mixing is continued,

[0110] 6. the mixture is devolatilized, preferably by heating to atemperature≧100° C.

[0111] The characteristic of this second method is associated with thefact that the process involves the co-addition of the particulatereinforcing filler and its compatibilizer. It is therefore conceivableto make a preblend of these two constituents, or, alternatively, tointroduce them concomitantly. The gradual incorporation in step 2′ maybe carried out continuously or in stages.

[0112] According to a variant of this second practical method ofimplementation,

[0113] 1″. the silicone oil is introduced,

[0114] 2″. the particulate filler—preferably silica—together with thefirst CA—preferably HMDZ—fraction and the water are gradually andsimultaneously incorporated into the oil,

[0115] 3. the mixing is continued,

[0116] 4. the second CA—preferably HMDZ—fraction is graduallyincorporated into the mixture obtained at 3,

[0117] 5. the mixing is continued,

[0118] 6. the mixture is devolatilized, preferably by heating to atemperature≧100° C.

[0119] According to one advantageous provision of the invention,corresponding to the case in which the reinforcing filler is silica andthe compatibilizer CA is HMDZ, a sufficient amount of HMDZ is used forthe content of Si(Me)₃ units on the surface of the silica to be ≧1Si(Me)₃ unit per mm² and preferably between 1 and 2 Si(Me)₃ units permm².

[0120] According to a third method of implementing the invention, theprocess to which it relates is characterized:

[0121] in that the first CA fraction is replaced, completely or partly,with at least one processing aid chosen from molecules and combinationsof molecules:

[0122] capable of interacting with the particulate filler, particularlywith silicon if a siliceous filler is used, to the detriment of thehydrogen bonds that this particulate filler establishes especiallybetween its own atoms and/or with those of the silicone oil,

[0123] and capable of being removed from the preparation mixture bydevolatilization,

[0124] and in that actions are taken to ensure that this processing aidis in the presence of water in the preparation mixture.

[0125] In accordance with this third method of implementation, it ispreferable for the processing aid to be readily removable from thepreparation mixture. For this purpose, it is beneficial for it to beeasily removed by devolatilization, for example by heating in a vacuumor in a gas stream. Under these conditions, it is clear that, asprocessing aid, molecules of low molecular weight will be preferred.

[0126] Advantageously, the processing aid is chosen from the groupcomprising:

[0127] silazanes, HMDZ being preferred;

[0128] difunctional, or preferably monofunctional, hydroxylatedsiloxanes;

[0129] amines, preferably ammonia and/or alkylamines, diethylamine beingparticularly preferred;

[0130] organic acids, formic and/or acetic acids being preferred;

[0131] and mixtures thereof.

[0132] As indicated above, the products more particularly selected asprocessing aids are those having a low molecular weight. This proves tobe the case especially for the amines and the organic acids mentionedabove.

[0133] With regard to the products employed in the process according tothe invention, it may be pointed out that, in the case of the siliconeoil, linear or cyclic, but more especially linear, polydiorganosiloxaneswill preferably be chosen.

[0134] With regard to the POS fluids (I), these will bepolydiorganosiloxane oils carrying an Si-alkenyl, particularly anSi-vinyl, group in and/or at the ends of the chain. In practice, mentionmay be made, for example, of α,ω-divinyl-terminated polydialkyl (methyl)siloxanes. Preferably, the POS (I) used for preparing the suspension isa vinyl POS (I) carrying at least two SiVi units per molecule,preferably at least three per molecule, when the POS (II) contains onlytwo SiH units per molecule.

[0135] As regards the POS (II), this is chosen frompolyorganohydrogenosiloxanes comprising at least two SiH units permolecule, preferably at least three, when the POS (I) comprises only twoSiVi units per molecule. In practice, mention may be made, for example,of polyalkyl(methyl)hydrogenosiloxanes or else branched hydrogenated POSfluids having trifunctional or tetrafunctional units and units carryingSiH.

[0136] The POS (III) may be a polydiorganosiloxane such as apolyalkylsiloxane, preferably a polydimethylsiloxane, havingtrimethylsilyl end groups.

[0137] The preferred silicone oils (I, II, III) essentially compriseR₂SiO units, the symbols R, which may be identical or different,representing C₁-C₄ (cyclo)alkyls which may or may not be halogenated, oraryl groups, which may or may not be substituted or halogenated.

[0138] By way of groups:

[0139] alkyl: mention may especially be made of methyl, ethyl, propyland butyl groups,

[0140] haloalkyl: mention may be made of 3,3-trifluoropropyl,

[0141] cycloalkyl: mention may be made of cyclohexyl,

[0142] aryl: mention may be made of the phenyl group.

[0143] Preferably, at least 85% of the groups R represent methyl groups.

[0144] The silica used in the process according to the present inventionis a reinforcing silica whose specific surface area is preferablybetween 50 and 400 m/g. These silicas may be precipitated silicas, butmore generally fumed silicas are employed. The fact that silica ispreferred does not exclude making use of other types of knownreinforcing filler.

[0145] The CA is preferably a silazane and even more preferably adisilazane. This is a product which is liquid under standard temperatureand pressure conditions (23° C./760 mmHg).

[0146] The viscosity of the suspension is one of the key parameterswhich govern the process according to the invention. Thus, in accordancewith one advantageous provision of the latter:

[0147] an alkenylated—preferably vinylated—silicone oil comprising atleast two Si-alkenyl groups per molecule, each preferably located at oneend of the chain, and having a dynamic viscosity at 25° C. not exceeding250 Pa.s, preferably not exceeding 100 Pa.s and more preferably stillnot exceeding 10 Pa.s, is employed,

[0148] a silica having a BET specific surface area of between 50 and 400m²/g and mixing conditions such that the dynamic viscosity at 25° C. ofthe suspension does not exceed 300 Pa.s, preferably does not exceed 250Pa.s and more preferably still does not exceed 200 Pa.s, are chosen.

[0149] Industrial Application

[0150] The purpose of the reinforcing filler/silicone oil suspensionprepared in accordance with the invention is for it to be used forobtaining liquid or pasty silicone compositions made of RTV siliconeelastomer, which compositions can be cured, preferably by polyaddition,in the ambient atmosphere and at a normal temperature or at a highertemperature.

[0151] Thus, according to another of these aspects, the presentinvention relates to a process for obtaining a silicone composition thatcan be cured by polyaddition, characterized in that it consists inmixing the following products:

[0152] A—a suspension as prepared according to the process as definedabove,

[0153] B—one or more POS fluids (I), as defined above,

[0154] C—one or more POS fluids (II), as defined above,

[0155] D—optionally, one or more POS fluids (III), as defined above,useful as diluent(s),

[0156] E—a catalytic system comprising a catalyst, preferably of theplatinum kind, and, optionally, an inhibitor or retarder.

[0157] According to a first variant of this process:

[0158] the composition is produced in the form of a two-component systemcomprising parts C₁ and C₂ which are intended to be brought into contactwith each other in order to produce an elastomer crosslinked bypolyaddition between the POS fluids (I) and (II),

[0159] and care is taken to ensure that only one of the parts, C₁ or C₂,contains some catalyst D and, optionally, one or other of the POS fluids(I) and (II).

[0160] According to a second variant of this process for preparingcurable liquid compositions, a one-component system is produced which isintended to be crosslinked in the ambient air and/or under the effect oftemperature.

[0161] These curable compositions, which are precursors of elastomers,may also comprise one or more functional additives F such as, forexample, a non-reinforcing filler formed by chalk, quartz powder,diatomaceous earth, mica, kaolin, aluminas or iron oxides. Theseoptional additives F may also consist of pigments, antiblocking agents,plasticizers or rheology modifiers, stabilizers or adhesion promoters.

[0162] The examples which follow illustrate:

[0163] the preparation of suspensions of reinforcing filler in siliconeoils in accordance with the invention,

[0164] the application of these suspensions as raw material forobtaining two-component curable compositions made of RTV siliconeelastomers,

[0165] and the evaluation of the viscoelastic properties of thesuspensions and the mechanical properties of the crosslinked elastomersobtained from the said suspensions.

[0166] Two methods of implementing the process of the invention aregiven in the examples. The latter also comprise comparative examples forpreparing suspensions in accordance with the prior art usingcompatibilization treatment methods of the “late” type and of the“early” type.

EXAMPLES Comparative Example 1: Late Treatment

[0167] Introduced into a 1.5 l arm mixer are 750 g ofα,ω-divinyl-terminated PolyDiMethylSiloxane (PDMS) oil having aviscosity of 0.6 Pa.s and 21 g of water. After homogenization, 321 g ofa fumed silica, characterized by its specific surface area of 300 m²/g,are added in portions over 70 minutes. After mixing for 120 minutes, 66g of hexamethyldisilazane are added over 90 minutes. A heating phasestarts 60 minutes later, during which, when the temperature reaches 80°C., the mixture is placed in a stream of nitrogen (250 l/h); the heatingcontinues until reaching approximately 155°, a steady temperature whichis maintained for 2 h. After cooling, 43 g of the vinyl-terminated oilare cooled and the suspension homogenized.

[0168] Starting from this suspension, a part A and a part B areformulated.

[0169] Part A contains:

[0170] 90.6 g of the suspension;

[0171] 1.58 g of the α,ω-divinyl-terminated PDMS oil described;

[0172] 5.48 g of an α,ω-dihydrogeno PDMS oil containing 1.9 meq SiH pergram of oil

[0173] 2.35 g of a polyhydrogeno PDMS oil having a viscosity of 30mPa.s, containing 1.6 meq SiH per gram of oil.

[0174] Part B contains:

[0175] 11 g of the suspension;

[0176] 29 g of the α,ω-divinyl-terminated PDMS oil described above;

[0177] 9.77 g of an α,ω-divinyl-terminated PDMS oil containing 0.05 meqVi per gram of oil;

[0178] 70 μl of a Karstedt catalyst containing 12% platinum;

[0179] 90 μl of divinyltetramethyldisiloxane;

[0180] 140 μl of tetravinyltetramethylcyclotetra-siloxane.

[0181] Parts A and B are mixed in a ratio of 100 to 10 and, afterdegassing, 2 mm thick plaques of elastomers are prepared. The mouldingsare cured in a ventilated oven for 1 hour at 150° C. The test piecesnecessary for measuring the mechanical properties are cut from theseplaques of cured elastomer.

Example 2: Early Treatment

[0182] The previous example is repeated except that the process startswith the mixer being charged with 750 g of α,ω-divinyl-terminated PDMSoil, 21 g of water and 66 g of hexamethylsisilazane [sic]. Afterstirring for 10 minutes, the silica is incorporated in portions over 30minutes; the mixing is continued for a further 120 minutes beforestarting the heating phase, which is the same as above.

[0183] The suspension is formulated as in Example 1.

[0184] Comparative Properties of the Suspensions and of the CuredElastomers According to Examples 1 and 2

[0185] The viscosity of the suspensions is measured by means of adynamic rheometer with a cone/plate geometry. The complex viscosity at 1Hz and at 1 Pa is taken as being representative of the viscoelasticbehaviour of the suspensions.

[0186] The mechanical properties are measured according to the standardsin force:

[0187] DIN 53505 for the hardness measurement

[0188] AFNOR T46002 for the breaking measurements. Dynamic Shore AStress at Elongation viscosity hardness break at break Pa · s pts MPa %Example 1 13 38 6.1 390 Example 2 3000 35 6.9 560

[0189] In the case of Example 1, the viscosity is low but the breakingproperties of the elastomer are moderate. With regard to Example 2, thishas better breaking properties, but at the price of having a highviscosity.

Example 3: Two-Step Treatment

[0190] Introduced into a 100 l arm mixer are 40 kg ofα,ω-divinyl-terminated oil having a viscosity of 2 Pa.s, 0.27 kg ofhexamethyldisilazane and 0.27 kg of water. After homogenization, 16.2 kgof a fumed silica characterized by its specific surface area of 200 m²/gare added in portions over 100 minutes. After 60 minutes of mixing, 1.9kg of hexamethyldisilazane are added over 60 minutes. A heating phase isstarted 120 minutes later, during which the mixture is placed in astream of nitrogen (30 m³/h); the heating continues until reachingapproximately 140°, a steady temperature which is maintained for 2 h.The suspension is then left to cool.

[0191] Starting from this suspension, a part A and a part B areformulated.

[0192] Part A contains:

[0193] 427 g of the suspension;

[0194] 10 g of a polyvinyl PDMS oil having a viscosity of 0.4 Pa.s,containing 0.11 meq Vi per gram of oil;

[0195] 25 g of trimethyl-terminated oil having a viscosity of 0.1 Pa.s;

[0196] 26.5 g of an α,ω-dihydrogeno PDMS oil, containing 1.9 meq SiH pergram of oil;

[0197] 11.3 g of a polyhydrogeno PDMS oil having a viscosity of 30mPa.s, containing 1.6 meq SiH per gram of oil.

[0198] Part B contains:

[0199] 180 g of the suspension;

[0200] 20 g of the methyl-terminated oil described;

[0201] 250 μl of a Karstedt catalyst containing 12% platinum;

[0202] 1 ml of tetravinyltetramethylcyclotetra-siloxane.

[0203] Parts A and B are mixed in a ratio of 100 to 10 and, afterdegassing, the test pieces necessary for measuring the mechanicalproperties are prepared as explained in Comparative Examples 1 and 2.

Example 4: Two-Step Treatment with Coaddition of the Reactants

[0204] The previous example is repeated except that the mixer is firstlycharged with 40 kg of α,ω-divinyl-terminated PDMS oil and 0.27 kg ofwater. After stirring for 10 minutes, the silica is incorporated inportions over 120 minutes at the same time as 0.27 kg ofhexamethyldisilazane, which is divided according to the portions ofsilica. After this phase of silica and hexamethyldisilazane coaddition,the process is continued as previously.

[0205] The suspension is formulated as in Example 3.

[0206] Properties of the Suspensions and of the Cured ElastomersAccording to Examples 3 and 4

[0207] The viscosity of the suspensions is measured by means of adynamic rheometer with a cone/plate geometry. The complex viscosity at 1Hz and at 1 Pa and the threshold stress, for which the elastic andviscous moduli are equal, are taken as being representative of theviscoelastic behaviour of the suspensions.

[0208] The mechanical properties are measured according to the standardsin force. Dynamic Threshold Shore A Tear viscosity stress hardnessstrength Pa · s Pa pts N/mm Example 3 165 25 34 21 Example 4 150 1 36 18

[0209] Examples 3 and 4 show that the elastomers prepared from theoil/silica suspensions obtained in accordance with the invention have arheology with a very low flow threshold and a moderate viscosity. Thisvery favourable rheological behaviour is accompanied by excellent tearstrength.

Example 5: Two-Step Treatment by Hexamethyldisilazane

[0210] Preparation of the Suspension

[0211] Introduced into a 7 l arm mixer are 2120 g of a mixture ofα,ω-divinyl-terminated oils having a viscosity of 1.5 Pa.s, 12.6 g ofwater and 12.6 g of hexamethyldisilazane. After homogenization, 765 g ofa fumed silica characterized by its specific surface area of 200 m²/gare added in portions over 110 minutes. Then, 80 g ofhexamethyldisilazane are added over 60 minutes. A heating phase starts120 minutes later, during which, when the temperature reaches 70° C.,the mixture is placed under vacuum; the heating continues until reachingapproximately 150° C., a steady temperature which is maintained for 1 h.The mixture is then cooled in a stream of nitrogen (≈250 l/h) and theapparatus is drained.

[0212] Starting from this suspension, a part A and a part B areformulated.

[0213] Part A contains:

[0214] 946.7 g of the suspension;

[0215] 42.7 g of an α,ω-dihydrogeno oil, containing 1.9 meq SiH per gramof oil;

[0216] 10.7 g of a polyhydrogeno oil, containing 1.6 meq SiH per gram ofoil.

[0217] Part B contains:

[0218] 22 g of the suspension;

[0219] 20 g of an α,ω-divinyl-terminated oil, containing 0.15 meq Vi pergram of oil;

[0220] 58 g of an α,ω-divinyl-terminated oil, containing 0.05 meq Vi pergram of oil;

[0221] 140 μl of a Karstedt catalyst containing 10% platinum;

[0222] 0.15 g of divinyltetramethyldisiloxane;

[0223] 0.35 g of tetravinyltetramethylcyclotetra-siloxane.

[0224] Parts A and B are mixed in a ratio of 100 to 10 and, afterdegassing, the test pieces necessary for measuring the mechanicalproperties are prepared.

Example 6: Treatment with an Acid First Step

[0225] The previous example is repeated except that the 12.6 gcorresponding to the first hexamethyldisilazane portion are replacedwith 3.6 g of formic acid. All the other operations are carried out aspreviously.

[0226] The suspension is formulated as in Example 5.

Example 7: Treatment with a Base First Step

[0227] Example 5 is again repeated, except that the 12.6 g correspondingto the first hexamethyldisilazane portion are replaced with 4.2 g ofaqueous ammonia containing 32% ammonia. All the other operations arecarried out as previously.

[0228] The suspension is formulated as in Example 5.

Example 8: Evaluation of the Comparative Properties of the Suspensionsof Examples 5 to 7

[0229] The viscosity of the suspensions is measured by means of adynamic rheometer with a cone/plate geometry. The following are taken asbeing representative of the viscoelastic behaviour of the suspensions:

[0230] the complex viscosity and its elastic and viscous components at 1Hz and at 1 Pa;

[0231] the threshold stress for which the elastic and viscous moduli areequal.

[0232] The mechanical properties are measured according to the standardsin force. Acid Base Example 5 Example 6 Example 7 Complex viscosity 4138 42 Elasticity factor 0.68 0.65 0.73 Flow threshold <1 <1 <1 Shore Ahardness 28 35 32 Elongation at break 700 400 610 Tensile strength 6.86.3 8.0 Tear strength 29 21 23

[0233] It may readily be seen that the rheological behaviour of thesuspensions is, in the three cases, typical of a product which flowswell and that these suspensions allow elastomers to be produced withgood mechanical properties.

1. Process for preparing a suspension of a particulate, preferablysiliceous, filler in a material formed by a silicone oil comprising:polyorganosiloxanes (POS fluids) of type (I) which carrySi-alkenyl—preferably Si-vinyl—functional groups capable of reactingwith the Si—H crosslinking functional groups of a POS fluid of type II,optionally, POS fluids of type (II) which carry Si—H crosslinkingfunctional groups capable of reacting with the Si-alkenyl functionalgroups of the POS fluids (I), and/or, optionally, POS fluids of type(III) which differ from the POS fluids (I) and (II), the said suspensionbeing able to be used, in particular, for producing siliconecompositions that can be cured by polyaddition, this process being ofthe kind of those in which the particulate filler is treated with theaid of a compatibilizing agent or compatibilizer (CA), characterized inthat it essentially consists in introducing some compatibilizer (CA)into the preparation mixture: on the one hand, before and/orsubstantially simultaneously with the contacting of at least part of thesilicone oil employed with at least part of the particulate filler used,this CA introduction taking place in one or more steps for a CA fractioncorresponding to a proportion of at most 8%, preferably at most 5% andeven more preferably at most 3% by dry weight with respect to the totalparticulate filler; and, on the other hand, after this POS/fillercontacting.
 2. Process according to claim 1, characterized in that itessentially consists: in mixing: 100 parts by weight of silicone oil 0to 5 parts by weight of water 20 to 80 parts by weight of particulatefiller consisting of silica 1 to 20 parts by weight of compatibilizer(CA) selected from silazanes taken by themselves alone or as a mixturethereof, preferably from disilazanes, hexamethyldisilazane which may ormay not be combined with divinyltetra-methyldisilazane beingparticularly preferred; in leaving the above to react, preferably withstirring, in heating the mixture obtained, choosing apressure/temperature pair so that at least some of the water and of thevolatile elements undergoes devolatilization; if necessary, in coolingthe mixture.
 3. Process according to claim 1 or 2, characterized inthat: all or some of the silicone oil, the water and all or some of theparticulate siliceous filler are mixed with a first CA fraction ofbetween 1 and 3% by dry weight with respect to the silica, a second CAfraction, representing between 10 and 15% by dry weight of silica, isincorporated into the mixture, optionally, the rest of the silicone oiland the rest of the silica are added, the mixture is allowed to react,preferably by continuing the mixing, the mixture is devolatilized,optionally, the devolatilized mixture is allowed to cool and,optionally, the suspension is completed with the rest of the siliconeoil.
 4. Process according to claim 3, characterized in that:
 1. amixture comprising the silicone oil, the water and the firstCA—preferably HMDZ—fraction is homogenized,
 2. the particulate filler,preferably silica, is gradually added to the mixture obtained at 1, 3.the mixing is continued,
 4. the second CA—preferably HMDZ—fraction isgradually incorporated into the mixture obtained at 3,
 5. the mixing iscontinued,
 6. the mixture is devolatilized, preferably by heating to atemperature≧100° C.
 5. Process according to claim 3, characterized inthat: 1′. the silicone oil and the water are homogenized, 2′. theparticulate filler—preferably silica—and, at the same time, the firstCA—preferably HMDZ—fraction are gradually incorporated into the mixtureobtained at 1,
 3. the mixing is continued,
 4. the second CA—preferablyHMDZ—fraction is gradually incorporated into the mixture obtained at 3,5. the mixing is continued,
 6. the mixture is devolatilized, preferablyby heating to a temperature≧100° C.
 6. Process according to claim 3,characterized in that: 1″. the silicone oil is introduced, 2″. theparticulate filler—preferably silica—together with the firstCA—preferably HMDZ—fraction and the water are gradually andsimultaneously incorporated into the oil,
 3. the mixing is continued, 4.the second CA—preferably HMDZ—fraction is gradually incorporated intothe mixture obtained at 3,
 5. the mixing is continued,
 6. the mixture isdevolatilized, preferably by heating to a temperature≧100° C.
 7. Processaccording to any one of claims 1 to 6, characterized in that the firstCA fraction is replaced, completely or partly, with at least oneprocessing aid chosen from molecules and combinations of molecules:capable of interacting with the particulate filler, particularly withsilicon if a siliceous filler is used, to the detriment of the hydrogenbonds that this particulate filler establishes especially between itsown atoms and/or with those of the silicone oil, and capable of beingremoved from the preparation mixture by devolatilization, and in thatactions are taken to ensure that this processing aid is in the presenceof water in the preparation mixture.
 8. Process according to claim 7,characterized in that the processing aid is chosen from the groupcomprising: silazanes, HMDZ being preferred; difunctional, or preferablymonofunctional, hydroxylated siloxanes; amines, preferably ammoniaand/or alkylamines, diethylamine being particularly preferred; organicacids, formic and/or acetic acids being preferred; and mixtures thereof.9. Process according to any one of claims 1 to 8, characterized in thatan alkenylated—preferably vinylated—silicone oil comprising at least twoSi-alkenyl groups per molecule, each preferably located at one end ofthe chain, and having a dynamic viscosity at 25° C. not exceeding 250Pa.s, preferably not exceeding 100 Pa.s and more preferably still notexceeding 10 Pa.s, is employed, and in that a silica having a BETspecific surface area of between 50 and 400 m²/g and mixing conditionssuch that the dynamic viscosity at 25° C. of the suspension does notexceed 300 Pa.s, preferably does not exceed 250 Pa.s, and morepreferably still does not exceed 200 Pa.s, are chosen.
 10. Process forobtaining a silicone composition that can be cured by polyaddition,characterized in that it consists in mixing the following products: A—asuspension as prepared according to the process as defined in any one ofclaims 1 to 9, B—one or more POS fluids (I), as defined in claim 1,C—one or more POS fluids (II), as defined in claim 1, D—optionally, oneor more POS fluids (III), as defined in claim 1, useful as diluent(s),E—a catalytic system comprising a catalyst, preferably of the platinumkind, and, optionally, an inhibitor.
 11. Process according to claim 10,characterized in that the composition is produced in the form of atwo-component system comprising parts C₁ and C₂ which are intended to bebrought into contact with each other in order to produce an elastomercrosslinked by polyaddition between the POS fluids (I) and (II) and inthat care is taken to ensure that only one of the parts, C₁ or C₂,contains some catalyst D and, optionally, one or other of the POS fluids(I) and (II).